Supplementary Materials

Capillary electrophoretic determination of heavy-metal ions using 11-mercaptoundecanoic acid and 6-mercapto-1-hexanol co-functionalized gold nanoparticle as colorimetric probe

Fig. S1 Image of the CE-AuNP based detection interface

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Fig. S2 Influence of buffer pH

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Influence of BGE concentration

The background electrolyte concentration imposed an effect on the separation and detection. The free heavy-metal ions are highly susceptible to wall-adsorption. However, acetate would suppress this detrimental effect via the acetate-metal ion complexing. Moreover, The moderate complexing strength made tuning the resolution possible by changing the buffer concentration(the complex constants of acetate to Cd2+, Pb2+ and Hg2+

are 2.51 × 102, 3.16 × 108 and 2.69 × 108, respectively [1]). High buffer ionic strength is helpful in suppressing the diffusion and electrodispersion of the metal ions during electrophoresis; but acetate at high concentration entering the reaction solution would deteriorate the detection.

Buffers at concentrations of 17-59.5 mM, at a constant acetic acid/NaOH of 10:7, were employed for the investigation. Fig. S3 depicts that the resolution between the peaks of Cd2+ and Pb2+ improved, and peaks of all the analytes became slimmer with increasing buffer concentration; but the detection sensitivity generally diminished. To reach a balance between separation and detection, a buffer composition of 25 mM acetic acid and 17.5 mM NaOH was selected for the ensuing studies.

Fig. S3 Influence of the concentration of the background electrolyteThe BGE concentrations (mM): a, 17; b, 25.5; c, 34; d, 42.5; e, 51; f, 59.5

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Effect of separation voltage

The impact of separation voltage was investigated in the range of 6 to 16 kV. The analysis time obviously shortened with increasing separation voltage. At 12 kV, the peaks of Cd2+ and Pb2+ were well resolved, and all the solutes could be quantitatively determined. Higher electrophoretic voltage did not improve the separation efficiency; on the contrary, notable peak broadening was observed at 16 kV (Fig. S4). We think this might be the consequences of not only the end column broadening which occurred under all the circumstances of this work, but also the Joule heating effect, which produced radial temperature gradient and became the dominant source for peak broadening at high separation voltage. Therefore, a separation voltage at 12 kV was chosen as the best compromise between separation time and resolution.

Fig. S4 Electropherograms of Cd2+, Pb2+ and Hg2+ under different separation voltages

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Fig. S5 The influence of pressure applied on the interface

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Influence of concentration of salt presented in the AuNP reaction solution

Ionic strength affected the detection sensitivity in biphasic ways. Introducing high concentration salts to the AuNP colloids would develop a thinner Debye screening length and therefore facilitate the aggregation of the AuNPs leading to sensitive detection [2]. On the other hand, chelating properties of ionic species changed with varying ionic strength of the solution [3]. The influence of ionic strength was investigated by adding NaNO3, NaCl and NaClO4 to the reaction solution separately to 6.7 mM. It was found that addition of salts lowered the detectability, and the lowest peak heights for the heavy metal ions were observed with addition of NaClO4 (Fig. S6). The salt anions might form ion pairs with the divalent heavy-metal ions, thus weaken their chelation with MUA on the AuNP surface. To obtain high sensitivity, no salts were added to the reaction solution.

Fig. S6 Effect of salt solutions in AuNP@MUA/MCH reagent solution on the detection sensitivity. Concentration of these salt solutions were 6.7 mM.

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Effect of the pH of the AuNP reaction solution

Influence of the reaction buffer pH on the detection was investigated in the range of 4.0-9.5, using 5 mM acetic acid/sodium hydroxide (for pH 4-5.5), 5 mM hexamethylenetetramine/nitric acid (pH 6.0), 5 mM sodium dihydrogen phosphate/sodium hydroxide (for pH 6.5-8.0) and 5 mM ammonium hydroxide/nitric acid (pH 8.5-9) as buffering components. We first test the influence of pH off-line, and found that the functionalized AuNPs aggregated at pH ≤ 4.5 or at pH ≥ 9.5. At low pH, physically adsorbed citrate and covalently bonded MUA were neutralized and, consequently, the repulsion between the AuNPs decreased, leading to aggregation of the nanoparticles. However, in the buffer of high alkalinity, trivalent citrate ions adsorbed on the AuNPs were replaced with monovalent hydroxide ion. Therefore, the zeta potential of the nanoparticles would change, resulting in aggregation [4]. Our results (Fig. S7) indicated that highest detection sensitivity could be achieved with AuNPs dispersed in reaction buffer at pH 5. However, the sensitivity with reaction solution adjusted by buffering components was lower than the one obtained with the bare AuNP colloids and the baseline of the latter was more stable, likely because the binding between the heavy-metal ions and the capping MUA was negatively affected by the ions introduced.

Fig. S7 Influence of the acidity of the reaction buffer

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The effect of flow rate of the AuNP solution

The solutes were resolved during CE in the separation capillary and were delivered to the reaction capillary, where they were mixed with AuNPs to initiate the aggregation. The detection sensitivity was dependent on the extent of aggregation, which was influenced by the concentrations of the heavy-metal ions and the content of the AuNPs available in the mixture. Therefore, the flow rate of the reaction solution should be optimized for the good performance. In the plot of peak areas against flow rate, Hg2+ and Pb2+ show a convex profile with a zenith at 2.2 × 10-4 mL/s (Fig. S8). At low flow rate, the concentration of AuNPs@MUA/MCH in the running buffer/reaction solution mixture was low, offering a relatively low sensitivity. Moreover, in the presence of high percentage of the running buffer, the nanoparticles aggregated, adsorbed on the capillary wall, as evidenced by the elevating baseline during electrophoresis. This phenomenon was alleviated with increasing flow rate of the reaction solution. More AuNPs@MUA/MCH delivered into the reaction capillary by the high-rate flow facilitated the sensitive detection, but dilution of the analytes by the reaction solution would become more significant with the increasing flow rate. According to the results, a flow rate of 2.2×10-4 mL/s was employed in the determination.

Fig. S8 The effect of flow rate on the detection sensitivity

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Capillary electrophoresis-capacitively coupled contactless conductivity detection (CE-C4D)Experiments were performed on a CE system consisting of a high-voltage power supply (Sanchuan High Tech., Tianjin, China) and an in-house made C4D detector. The detector was described previously [5]. Briefly, two stainless steel needles with dimensions of 5 mm × 0.5 mm i.d. were employed, one acting as actuator electrode and another as pick-up electrode. The electrodes were separated by a copper foil and were shielded by a grounded copper housing. During measurement, an oscillation voltage (sinusoidal wave in a frequency range from 1 Hz to 2 MHz, at an amplitude between 0 and 20 V peak-peak voltage) was applied on the actuator electrode by A VC2002 function generator (Shengli Cooperation, Shenzhen, Guangdong, China), and the signal reflecting the conductivity of the gap between the electrodes was collected by a data acquisition unit (CT22, Qianpu, Shanghai, China) and processed with HW2000 chromatography station (Qianpu). A 60 cm (50 cm in effective length) 75 µm i.d. bare fused-silica capillary (Ruifeng Photoconduction Fiber, Hebei, China) was employed for separation.

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Fig. S9 CE-AuNP/ABCD detection of heavy-metal ions in facial masks and lotion.

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References[1] J.A. Dean, Lange's handbook of chemistry, 15th Edition ed., McGraw-Hill, Inc., New York, 1999.[2] J.K. Lim, S.W. Joo, Gold nanoparticle-based ph sensor in highly alkaline region at ph > 11: Surface-enhanced Raman scattering study, Appl.Spectrosc. 60 (2006) 847-852.[3] B. Svensson, B. Jonsson, M. Fushiki, S. Linse, Ionic-strength effects on the binding constant of calcium chelators - experiment and theory, J. Phys. Chem. 96 (1992) 3135-3138.[4] S.H. Wu, Y.S. Wu, C.H. Chen, Colorimetric sensitivity of gold nanoparticles: Minimizing interparticular repulsion as a general approach, Anal. Chem. 80 (2008) 6560-6566.[5] Z. Tian, W. Qin, Capillary electrophoretic separation of anions in dimethylformamide-acetic acid medium, Anal. Methods 6 (2014) 5353-5359.

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